Stabilized pressure-hydrated magnesium hydroxide slurry from burnt magnesite and process for its production

ABSTRACT

A stabilized, pressure-hydrated magnesium hydroxide slurry and a process for its production from burnt magnesite are described. According to an embodiment of the invention, a mixture comprising burnt natural magnesite and water is pressure hydrated to provide a pressure hydrated slurry. The pressure hydrated slurry is then deagglomerated. If desired, chloride ions and cationic polymer can be added to further stabilize the slurry.

FIELD OF THE INVENTION

This invention concerns a method of producing a pumpable, stablemagnesium hydroxide slurry by pressure hydration and stabilization ofburnt natural magnesite.

BACKGROUND OF THE INVENTION

Magnesium hydroxide in slurry form is useful as a pumpable source ofmagnesium hydroxide for various chemical processes, including but notlimited to the following: (1) pH adjustment, including waste acid andacidic wastewater neutralization; (2) wastewater treatment, includingprecipitation of heavy metal contaminants; (3) scrubbing andneutralization of acidic vapors in flue gases or process off-gases; and(4) production of specialty magnesium compounds (e.g. MgSO₄, MgNO₃,MgCl₂, etc.) as a source of magnesium.

By providing magnesium hydroxide in slurry form of at least moderatequality, capital extensive and potentially dangerous (due to thehydration exotherm) MgO powder slaking operations are eliminated for theend user. Magnesium hydroxide slurries of such quality have additionaladvantages including the ability to be easily handled and stored, andthe ability to be reliably dosed to chemical processes as desired. Infact, such magnesium hydroxide slurries can be transported to the pointof application and stored for time periods of days to several weeksunder intermittent to constant agitation without incurring adverseeffects such as settling of solids and excessive viscosity.

Lower quality atmospheric hydrated magnesium hydroxide products producedfrom burnt natural magnesite are common, but in many cases do not havethe above desirable characteristics and can cause numerous processingdifficulties including the following: product inhomogeneity; obstructionof transport pipelines, valves, processing equipment and storageequipment; formation of an impacted bed of non-pumpable magnesiumhydroxide solids at the bottom of storage tanks and process vessels:inconsistent or insufficient feed rate to processes: excessive energycosts for transporting/pumping the product; and high maintenance costsfor systems incorporating the product.

At present, however, the only moderate to high quality magnesiumhydroxide slurries available are believed to be synthetic magnesiumhydroxide products produced from the soluble magnesium present in brinefields and seawater. For economic reasons, these synthetic magnesiumhydroxide slurries are generally produced in close proximity to sourcesor brine or seawater.

SUMMARY OF THE INVENTION

In view of the above, there is presently a need for a magnesiumhydroxide slurry produce of at least moderate quality which can beproduced from geographical sources of natural magnesite ore that are notnecessarily linked to the brine fields and coastal areas currentlynecessary for the production of synthetically produced magnesiumhydroxide products.

At the time of the invention, it was not believed to be economicallyfeasible to produce a magnesium hydroxide slurry from burnt naturalmagnesite that would be or sufficient stability to be stored for longperiods of time or transponed over long distances, without settling intoan impacted bed.

Nonetheless, the present inventors have developed a process forproducing a moderate quality, pumpable, stabilized aqueous slurry ofmagnesium hydroxide that is produced from burnt natural magnesite. Inparticular, a process for the production of a stabilized magnesiumhydroxide slurry has been developed in which a mixture containing burntnatural magnesite and water is pressure hydrated to provide a precursorypressure hydrated magnesium hydroxide slurry. A key aspect of thestability of the final product is based on the synergistic reactionbetween two stabilizing additives: chloride ions and cationic polymer.These additives, along with the precursory pressure hydrated magnesiumhydroxide slurry are processed through a disperser to provide astabilized slurry.

The magnesium hydroxide slurry produced by the method of the presentinvention has many advantages, including but not limited to thefollowing: (1) hydration time is significantly reduced compared toatmospheric hydration, resulting in reduced production cycle time; (2)the magnesium hydroxide particle size distribution of the product of theinvention is smaller than that of atmospheric hydrates, contributing toimproved slurry stability; (3) maintenance costs are reduced compared toatmospheric hydrates since the pressure hydration step produces hydrateparticles that are easier to deagglomerate and stabilize thanatmospheric hydrate particles; (4) pumping and agitation costs are lowrelative to atmospheric hydrates; (5) cleanout costs for removingsettled or impacted solids from the bottom of storage or processvessels, from pipelines, from transportation vessels (e.g., tanktrucks), and so forth are lower than atmospheric hydrates; (6) systemsand piping networks already in place are amenable to conversion fromanother alkali hydroxide (such as that produced from lime) to magnesiumhydroxide, or from a premium quality magnesium hydroxide to a moderatequality magnesium hydroxide; (7) the end user may avoid modifying theapplication system layout and operating procedures, or upgradingexisting equipment to facilitate use of the magnesium hydroxide slurryfrom burnt magnesite; and (8) the process is not necessarily linked toparticular geographic areas for economic delivery of raw materials,allowing facilities employing the process to be located strategically tosupply targeted markers with the advantage of relatively low productshipping costs.;

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an Arrhenius plot based on the atmospheric hydration reactionof burnt natural magnesite.

FIG. 2 is a schematic diagram of a possible embodiment of the method ofthe present invention.

DETAILED DESCRIPTION

According to an embodiment of the invention, the principal processingrequirements for the production of a stable, pumpable, moderate qualitymagnesium hydroxide slurry from burnt natural magnesite are (1) pressurehydration; and (2) stabilization.

As used herein, the term "moderate quality" slurry encompasses thefollowing characteristics: (1) percent solids by weight is at least 50%,preferably 55-65%; (2) Brookfield Viscosity at room temperature is50-900 centipoise (cps), preferably 50-300 cps; (3)pourability/flowability is such that greater than 80% by weight,preferably greater than 90% by weight, of sample pours off after 7 daysof undisturbed gravity settling; (4) after 7 days of undisturbed(unagitated) gravity settling, water separation is less than 1 inch,preferably less than 1/2 inch, with the height of water separation beingmeasured in a standard, cylindrical 8 oz. poly channel bottle (2 in.OD×53/8 in. height); and (5) settled solids are readily re-suspendablewith minimal agitation intensity. For many applications,resuspendability is much more critical than long-term slurryhomogeneity, since intermittent agitation can be provided at the pointof application.

PRESSURE HYDRATION

In endeavoring to produce a magnesium hydroxide product from burntmagnesite, the inventors have unexpectedly discovered that pressurehydration of burnt magnesite, preferably in the presence of chlorideions, can result in a moderate to high quality product at competitivecost.

The magnesite of the present invention is preferably obtained fromnatural sources. Large deposits of natural magnesite arc found, forexample, in the United States, Canada, China, Korea, Australia, Greece,Spain, Brazil, Turkey, Austria, Czechoslovakia, Russia, Ukraine,Yugoslavia, Italy, India, Nepal and South Africa.

The chemical composition of burnt natural magnesite preferably comprisesabout 85 to 99 weight % MgO (ignited basis), and is derived by thethermal decomposition of magnesite ore (MgCO₃) to form magnesium oxide(MgO) and carbon dioxide (CO₂). Typical chemical composition of MgO andvarious major impurities of the preferred burnt magnesite ore of thepresent invention are summarized in Table 1A.

                  TABLE 1A                                                        ______________________________________                                        RANGE OF TYPICAL VALUES                                                       ______________________________________                                        MgO, weight % (ignited 86.0-98.0                                              basis)                                                                        CaO                    0.70-4.0                                               SiO.sub.2              0.25-11.0                                              Fe.sub.2 O.sub.3       0.07-4.5                                               Al.sub.2 O.sub.3       0.06-0.85                                              ______________________________________                                    

The burnt natural magnesite used in the practice of the presentinvention is preferably as finely divided as is commercially feasible,preferably passing through a 20 mesh screen, more preferably passingthrough a 100 mesh screen.

The burnt natural magnesite used in the practice of the presentinvention can be produced using kilns of various designs includingstatic shaft kilns, rotary kilns. Herreshoff kilns, step kilns, and soforth.

Hydration of the burnt natural magnesite comprises the reaction betweenthe magnesium oxide, MgO, and water to produce magnesium hydroxide,Mg(OH)₂. At atmospheric pressure and at temperatures less than or equalto 212° F., however, completely hydrating burnt natural magnesite takesas long as a week. The reactive portion hydrates relatively quickly attemperatures attainable at atmospheric pressure, but the less reactiveportion hydrates extremely slowly under such conditions.

By contrast, the inventors have found that pressure hydration providesan expedient method of hydrating burnt natural magnesite, apparently byovercoming pore diffusion and mass transfer resistances as discussed inExample 3 below. Regardless of the mechanism by which hydration occurs,super-atmospheric pressures and corresponding saturation temperaturesdramatically increase the rate of hydration of the subject burnt naturalmagnesite. For example, hydration time is reduced from periods on theorder of days at atmospheric pressure to period on the order of hours orminutes at super-atmospheric pressures.

In conducting pressure hydration, it is preferred to couple thepressurized hydration equipment with a means of recovery heat to enablethe re-use of heat evolved during hydration (an exothermic reaction).For example, if batch processing is selected, later batches can bepre-heated with energy evolved from present batch, using recuperativeheat transfer. The exotherm can also be used effectively to overcome theactivation energy barrier of the hydration reaction and speed uphydration. Of course, energy can also be recovered if a continuousprocessing mode is selected.

Depending on volume, operating pressure, material of construction andprocessing mode (batch or continuous), the hydration vessels selectedcan be, for example, vertical, agitated pressure vessels (better suitedto batch processing), horizontal, paddle-agitated cylindrical vesselssloped to improve vessel discharge (better suited to continuousprocessing) and so forth.

Hydration pressures are preferably 1 to 150 psig, more preferably 25 to100 psig, with the actual pressure selected based on capital costs,energy costs and so forth.

STABILIZATION

The magnesium hydroxide slurries of the invention are preferablystabilized by subjecting the hydrated slurry to chemical and mechanicaltreatment.

Mechanical treatment of the hydrated slurry of the invention itpreferably performed to deagglomerate the product and disperse anydesired additives. Commercially-available equipment which can be usedfor this purpose include homogenizers and high-speed shear mixers suchas tandem shear pipeline mixers, high-speed dispersion blades,homogenizer/reactors, in-line static mixers, agitated hold-up tanks andother suitable devices.

Chemical stabilization of the hydrated slurry of the invention ispreferably accomplished by the addition of chloride ions and cationicpolymer. As shown below in the Examples, chloride ions and cationicpolymer provide a product of surprisingly high stability that is readilysuspendable with mild, intermittent agitation. Any solid settlement ofthe magnesium hydroxide slurries of the present invention is soft, asopposed to a tacky, impacted bottom solid that is formed without theaddition of either of these materials.

.[.Prior to the present invention, it was known that the addition ofchlorides to magnesium hydroxide slurries would contribute to thestability and suspendability of the product..]. In fact, chlorides areintentionally removed from brine and seawater based magnesium hydroxideslurries by washing. Chloride ions are known to accelerate variouscorrosion processes. Nevertheless, chlorides ions are intentionallyadded to the magnesium hydroxide slurry in accordance with the presentinvention, resulting in an unexpected increase in stability andresuspendability.

Preferred sources of chloride ions for the practice of the inventioninclude calcium chloride, sodium chloride, aluminum chloride, magnesiumchloride potassium chloride, ammonium chloride, hydrated species such asCaCl₂.H₂ O, CaCl₂. 2H₂ O, CaCl₂.6H₂ O, MgCl.6H₂ O, AlCl₃.6H₂ O and soforth. The most preferred chloride salt is calcium chloride, which canbe added, for example, as a CaCl₂ brine or as ground prills no dryCaCl₂.

According to an embodiment of the invention, the chlorides are chargedto the hydrator prior to the hydration reaction. The chlorides tend tocause a larger particle size distribution exiting the hydrator, but thelarger agglomerates can be deagglomerated to produce a slurry productthat is more stable than one without chlorides. For example, apressure-hydrated slurry with a larger hydrate particle sizedistribution can be deagglomerated by passing through one of themechanical devices discussed above.

Concentrations of chloride ions used in the practice of the presentinvention preferably range from 0.01 to 2.5 weight % of dry MgO, morepreferably 0.2 to 0.5 weight % of dry MgO. It is noted that burntmagnesite reactant is inherently low in chlorides, with chlorideconcentrations being on the order of 0.001 to 0.01 weight % of dry MgO.

Along with the chloride ions, addition of cationic polymer stabilizesthe slurry by synergistic action and produces a readily suspendablematerial.

Cationic polymers are known to be useful for adjusting viscosity towithin an acceptable range. .[.However, prior to the present invention,it was not known that cationic polymers could be used to improve thestability and resuspendability of magnesium hydroxide slurries..].Indeed, the fact that cationic polymers are used to decrease viscositysuggests that the resulting product will exhibit a decrease instability. This is because in general solids are known to settle fasterin less viscous solutions than in more viscous solutions. However, theinventors have found, to the contrary, that stability .Iadd.of apressure hydrated magnesium hydroxide slurry .Iaddend.can be increasedby such additives.

The cationic polymer is preferably added after hydration, as cationicpolymers are generally unstable at the hydration temperatures preferredfor the practice of the invention. Preferred polymers include cationicspecies particularly preferred are polyamine polymers. The preferredanion for the polymer is the chloride ion.

Preferred concentrations of the polymer range from 0.01 to 2.0 wt. %slurry weight basis, more preferably 0.1 to 0.20 wt. % slurry weightbasis. Ideally, the polymer should yield a product with a viscosity of50-900. preferably 50-300 centipoise.

It will be clear to the one of skill in the art that a near-infinitenumber of processing facilities can be constructed to carry out thepresent invention. One such facility is shown in FIG. 2. The varioushydration and stabilization apparatus shown in FIG. 2 reflectcommercially-available equipment of standard construction.

Referring now to FIG. 2, burnt natural magnesite stored in storage silo11 passes through valve 22 to weigh belt feeder 12. The burnt naturalmagnesite introduced by way of weigh belt feeder 12 is then mixed withwater from process water supply A in mix/wet-in tank unit 14 withaccompanying piston pump 13. An input of chloride ions F is fed into themixture of burnt natural magnesite and water emerging from themix/wet-in tank unit 14 and the resulting mixture enters hydrator 15,where an input of steam B is introduced to achieve the appropriatehydrating temperature and pressure. Depending on whether the process isbeing started up or is operating continuously, the heat of reaction maybe sufficient to maintain hydrator temperature and pressure, obviatingthe need for outside steam. The resulting hydrated slurry then entersflash vessel 17, where the steam that is flashed off is passed throughreflux condenser 16. The steam is condensed with an input of cold waterC1 which emerges as cold water output C2. The condensed steam isreturned to the hydrator to reuse its sensible heat. The hydrated slurryis then pumped by pump 19 through heat exchanger 18, where the slurry iscooled by accompanying cold water input C1, which emerges as cold wateroutput C2. The cooled slurry emerging from heat exchanger 18 then entersdispersator surge tank 20 where it is mixed with make-up/cooling waterfrom water input stream D to adjust percent solids into the target rangeas necessary. The slurry then exits the dispersator surge tank 20 whereit is combined with cationic polymer from stabilizing polymer inputstream E, and the resultant mixture is introduced into a firstdispersator 21. The slurry to emerging from the first dispersator 21passes through interpass surge tank 23 and enters second dispersator 21.The stabilized slurry finally emerges as output stream H.

Many additional processing schemes will become quickly apparent to thoseskilled in the art. For example, the scheme in FIG. 2 is a continuousprocessing scheme, but batch and semi-batch processes will quicklybecome apparent. As another example, the scheme shown in FIG. 2comprises two separate dispersators, but several passes can be madethrough a single dispersator if desired.

The various sources of chloride ions, cationic polymers, processingequipment, processing parameters, and so forth, to be used in aparticular application of the present invention can be evaluated, forexample, using the 7 and 14 day pour tests discussed below to determinethe optimal process for the desired application.

The invention will be further clarified by consideration of thefollowing examples, which are intended to be purely exemplary of the useof the invention.

EXAMPLES

Example 1

Table 1B summarizes a laboratory analysis of Chinese burnt naturalmagnesite used in the Examples. Of course, the range of percent MgO andmajor chemical impurities can vary somewhat with burnt natural magnesitefrom other sources, or even within burnt magnesite taken from a singlesource. Table 1A above lists the various ranges of percent MgO and majorchemical impurities that can be expected for a given sample of burntnatural magnesite.

                  TABLE 1B                                                        ______________________________________                                        Chemistry                                                                     MgO, weight % (ignited basis)                                                                      97.16                                                    CaO, weight %        1.18                                                     SiO.sub.2, weight %  0.49                                                     Fe.sub.2 O.sub.3, weight %                                                                         0.88                                                     Al.sub.2 O.sub.3, weight %                                                                         0.29                                                     Cl, weight %         0.03                                                     SO.sub.3, weight %   0.36                                                     Screens, % passing                                                            100 mesh             99.9                                                     200 mesh             99.8                                                     325 mesh             97.3                                                     Median Particle      5.94                                                     Size, microns                                                                 ______________________________________                                    

Samples of magnesium hydroxide slurry containing 55 to 57 weight %solids were produced from the above burnt natural magnesite as follows:

(1) a laboratory autoclave was charred with the burnt natural magnesite,water, and a 23 weight % CaCl₂ solution in relative amounts necessary toprovide 0.5 weight % chloride concentration (MgO basis) and a 55-57weight % (slurry basis) magnesium hydroxide solids product.

(2) the mixture of step (1) was pressure hydrated in the autoclave byheating and pressurizing the autoclave to 100 psig (328° F.). holding at100 psig for 10 minutes, then cooling the sample to near ambienttemperatures for safe handling in the laboratory. In field practice,however, as little as 20° F. of subcooling may be needed to preventdamage to downstream equipment. The heat-up period was approximately 50minutes and cooldown took approximately 80 minutes. Hence, totalresidence time in the autoclave was 140 minutes.

(3) 1000 ppm (heat basis of polymer, added on a slurry weight basis) ofcationic polymer (Nalco 91DA054, an aqueous solution of polyamine) wasadded to the product of step (2) while blending it low speed with a Shardispersion blade.

(4) the slurry was then processed through an APV Gaulin Model 15MR-8TAlaboratory homogenizer at 1500 psig and samples were withdrawn afterone, two, and three passes.

Samples retained after hydration (without cationic polymer andhomogenizer processing) are compared on the basis of magnesium hydroxideparticle size in Table 2 with samples of product that had been providedwith cationic polymer and processed with the homogenizer. Polymer wasadded only to the stabilized products. Particle size distributions inall cases were measured with a Micromeritics Sedrigraph 5100.

                  TABLE 2                                                         ______________________________________                                                       Chloride and Polymer                                                     # of homogenizer passes                                                       @ 1500 psig                                                                   Chloride                                                                      Only                                                                          0 passes                                                                             1 pass    2 passes                                                                              3 passes                                   ______________________________________                                        weight % solids                                                                           56.0     57.0      55.0  55.6                                     (measured by oven                                                             method)                                                                       cumulative mass                                                                           Equivalent spherical                                              weight % finer                                                                            diameter, microns                                                 than equivalent                                                               spherical diameter                                                            90          57.2-67.0                                                                              39.3      36.6  39.7                                     70          40.3-45.6                                                                              23.0      21.4  21.4                                     50          31.4-37.2                                                                              13.9      12.0  11.5                                     30          22.0-28.6                                                                              3.8       2.8   2.6                                      10           0.2     0.2       0.3   --                                       ______________________________________                                    

It is apparent from Table 2 that the particle size distribution isimproved with respect to slurry stability by passing through thehomogenizer.

The above samples were subjected to 7 and 1.1 day stability testing andthe results summarized in Tables 3 and 4 respectively. Pour tests wereconducted using standard, cylindrical 8 oz. poly channel bottles (2 in.OD×53/8 in. height). For many applications, resuspendability is muchmore critical than long-term slurry homogeneity, because intermittentagitation can be provided at the point of application. The limiting timeperiod for slurry homogeneity is time of transportation to the site ofapplication, which can range from as little as 2 hours to several days.

                  TABLE 3                                                         ______________________________________                                                  Weight %             Viscosity                                                poured       Water   after 7-day                                    # of passes                                                                             after 7      Split,  pour & mix                                     @ 1500 psig                                                                             days         in.     (centipoise)                                   ______________________________________                                        0         0*                                                                  1         84.8         7/16    45                                             2         92.7         5/16    54                                             3         90.5         1/4.sup.                                                                              66                                             ______________________________________                                         *Solids form a very tacky, dense sediment during the sample cooling           period.                                                                       Only the water split pours off.                                          

                  TABLE 4                                                         ______________________________________                                                 Wt. %                     viscosity                                           poured                    after                                               after   Water     Bottom  14 day                                     # of passes                                                                            14      split,    solids  pour & mix                                 @ 1500 psig                                                                            days    inches    description                                                                           (cps).sup.(2)                              ______________________________________                                        1        35.9    21/8.sup. Tacky   61.0                                       2        58.9    11/16     Soft to 64.5                                                                  bottom                                             3        59.7    15/16     Soft to 58.8                                                                  bottom                                             ______________________________________                                         Note:                                                                         .sup.(2) Brookfield Viscometer, RVT, # 1 spindle @ 100 rpm.              

The results presented in Tables 3 and 4 show that two stabilizationpasses resulted in soft settlement even after 7 and 14 days ofundisturbed settling. Without adequate stabilization, the magnesiumhydroxide solids settle and form a very tacky, impacted settlement.

The presence of chlorides is also important to the final product. Inpour tests performed on slurry samples that were not dosed with thechlorides, only the clear layer of water separation pours off afterseven days of static settling. In most cases where chloride is notadded, a tacky, non-pourable layer of bottom solids form is within thefirst twenty-four hours of undisturbed settling. Obviously thiscondition renders the magnesium hydroxide unusable.

The effect of chlorides on particle size distributions for magnesiumhydroxide made with and without 0.5 weight % (dry MgO basis) chloridesadded to hydrator prior to hydration was also investigated. The resultsare summarized in Table 5 for samples as they emerged from the hydrator(no polymer treatment and no homogenization).

Table 5 clearly shows that the addition of chlorides increases theparticle size of the resultant Mg(OH)₂ product apparently byagglomeration. However, particle size of the chloride-treated sample asnearly returned to non-chloride-treated levels after only one passthrough the dispersator it 1500 psig (see Table 2, column 3).

                  TABLE 5                                                         ______________________________________                                                         Equivalent spherical                                         Cumulative mass %                                                                              particle diameter,                                           finer than       microns                                                      equivalent spherical                                                                           without  with                                                particle diameter                                                                              chlorides                                                                              chlorides                                           ______________________________________                                        90               32.54    57.2-67.0                                           70               17.00    40.3-45.6                                           50               6.42     31.4-37.2                                           30               0.77     22.0-28.6                                           10               0.33     0.2                                                 ______________________________________                                    

Example 2

Magnesium hydroxide slurry samples were prepared using she sameformulation as in Example 1, but a different mechanical dispersator wasused. A Silverson High Shear Mixer (laboratory batch mixer) was used inplace of the APV Gaulin 15MR-8TA Homogenizer. Different replaceablemixing heads were used on the Silverson mixer to determine those thatproduce an acceptable magnesium hydroxide slurry product. 250-ml slurrysamples were processed for 5 minutes with the laboratory batch mixer.The product samples are described below in Tables 6 and 7. Percentsolids vary in these samples from 57 to 62 weight %. In both tables,samples with and without cationic polymer have been processed with thehigh speed mixer using the various to mixing heads. The product sampleswere characterized by several measures including percent solids and7-day pour tests. Moreover, after 5 months of undisturbed settling, thesamples were lightly agitated to re-suspend the settled solids and theviscosity was measured by Brookfield viscometer, is Finally, theparticle size distribution was also investigated. As can be seen forsome of the samples, the qualitative determination of resuspendability,as determined by probing the bottom solids with a glass stirring rod,was favorable despite pourable weight fractions less than 80%.

                                      TABLE 6                                     __________________________________________________________________________                             chloride addition                                            chloride addition only                                                                         with cationic polymer                                        7-day                                                                             water                                                                             weight                                                                            Viscosity                                                                          7-day                                                                             water                                                                             weight                                                                            Viscosity                                        pour,                                                                             split,                                                                            %   after 5                                                                            pour,                                                                             split,                                                                            %   after 5                                  Mixing Head                                                                           wt. %                                                                             inches                                                                            solids                                                                            mo., cps.sup.(1)                                                                   wt. %                                                                             inches                                                                            solids                                                                            mo., cps.sup.(1)                         __________________________________________________________________________    Square- 55.8.sup.(2)                                                                      3/8 59.3                                                                              110  67.3.sup.(3)                                                                      5/8 59.9                                                                              110                                      hole high shear                                                               screen                                                                        General purpose                                                                       88.1.sup.(4)                                                                      13/16                                                                             58.5                                                                               98  79.0.sup.(5)                                                                      11/8                                                                              58.0                                                                              108                                      disintegrating                                                                head                                                                          Vertical                                                                              57.9.sup.(6)                                                                      11/16                                                                             61.4                                                                              117  57.1.sup.(7)                                                                      11/16                                                                             61.1                                                                              124                                      slotted                                                                       disintegrating                                                                head                                                                          Fine    27.7.sup.(8)                                                                      3/4 61.2                                                                              178  92.6.sup.(9)                                                                      3/4 57.8                                                                              102                                      perforation                                                                   emulsion screen                                                               __________________________________________________________________________     Table 6 notes and comments:                                                   (1) After undisturbed settling for 5 months, samples were lightly             agitated. Viscosity was then determined by Brookfield Viscometer, RVT, #2     spindle at 100 rpm.                                                           (2) Settled solids layer (21/4 inches) very pourable and easily               resuspendable upon application of slight shear; relatively low pour           fraction believed to be due to thixotropy or Binghamplastic rheology. Ver     slight shearing/agitation makes settled solids pourable.                      (3) Bottom slurry layer (113/16 inches) very pourable upon application of     slight shear with no tacky buildup on bottom. Resuspendable.                  (4) Bottom slurry layer (11/16 inch) readily pours upon application of        slight shear; very little tacky buildup on bottom. Very slight thickening     of settled solids layer.                                                      (5) Bottom slurry layer (13/16 inch) readily pours upon application of        slight shear; very little tacky buildup on bottom.                            (6) Bottom slurry layer (211/16 inches) readily pours upon application of     slight shear. Slight thickening around walls of container, but no tacky       buildup on bottom.                                                            (7) Bottom slurry layer (23/8 inches) very soft with no tacky buildup on      bottom; readily resuspended and made pourable thickening on walls of pour     test container.                                                               (8) Bottom slurry layer (37/8 inches) very soft with slight thickening on     walls of pour test container; readily resuspended upon application of         slight shear/agitation.                                                       (9) Bottom slurry layer (1/2 inch) pours very readily with slight sheaar.     No tacky buildup on bottom.                                              

                                      TABLE 7                                     __________________________________________________________________________            w/out cationic polymer                                                                       with cationic polymer                                          added before stabilization                                                                   added before stabilization                                     cumulative weight %                                                                          cumulative weight %                                            finer than table entry                                                                       finer than table entry                                         particle size  particle size                                          Silverson                                                                               90                                                                               70                                                                               50                                                                               30                                                                               10                                                                               90                                                                               70                                                                               50                                                                               30                                                                               10                                       Mixing Head                                                                           particle size, microns                                                                       particle size, microns                                 __________________________________________________________________________    Square hole                                                                           35.21                                                                            20.94                                                                            12.72                                                                            4.30                                                                             0.44                                                                             70.08                                                                            11.96                                                                             6.14                                                                            2.28                                                                             1.00                                       high shear screen                                                             General purpose                                                                       23.90                                                                            13.03                                                                             7.20                                                                            2.06                                                                             0.35                                                                             26.06                                                                            11.36                                                                             5.74                                                                            0.77                                                                             0.35                                       disintegrating                                                                head                                                                          Vertical slotted                                                                      35.61                                                                            23.44                                                                            15.40                                                                            8.34                                                                             0.73                                                                             35.94                                                                            23.13                                                                            15.33                                                                            8.45                                                                             0.62                                       disintegrating                                                                head                                                                          Fine    35.46                                                                            22.57                                                                            14.70                                                                            7.91                                                                             0.52                                                                             35.87                                                                            22.66                                                                            14.55                                                                            7.29                                                                             0.40                                       perforation                                                                   emulsion screen                                                               __________________________________________________________________________

In the eight cases shown in Tables 6 and 7 above, it is apparent fromqualitative probing and the 5-month viscosity measurements (5 monthsundisturbed settling followed by a single, light agitation), that withintermittent and moderate (3 to 5 on an agitation intensity scale of 1to 10) agitation to resuspend settled solids, the magnesium hydroxideslurry will be acceptable for many applications. It is emphasized thatthe pour test, because of its quantitative nature, does not acknowledgeslurry that is pumpable with bottom solids that readily resuspend aftera light shear force is applied to overcome, for example, Binghamplasticity or zero-deformation rate resistance to flow. Table 7 alsoclearly shows the beneficial results obtained upon the addition ofcationic polymer. For example, compare the 7 day pour percentages forsamples with polymer (92.6 weight %) and without polymer (27.7 weight %)when using the fine perforation emulsion screen in connection with theSilverson Mixer. The particle size distributions summarized in Table 7demonstrate that the above-noted increase in stability observed uponaddition of cationic polymer is not due to a change in the particle sizedistribution of the resulting slurry.

Example 3

Laboratory studies were conducted to investigate the apparent kineticsof the hydration reaction of burnt magnesite. Although details of thereaction mechanism could not be established with certainty,rate-limiting phenomena can be recognized based on approximateactivation energies. Results of these tests can be used to contrast thedisadvantages of atmospheric hydration with the efficacy of pressurehydration. The results of these studies ire summarized in the Arrheniusplot of FlG. 1.

From FIG. 1, it was determined that the slope to the left of thevertical solid line was about -350° K., which corresponds to an apparentactivation energy of about 2.9 kJ/g-mol. Since the apparent activationenergy was less than about 4 kJ/g-mol, it is believed that porediffusion and mass transfer control the reaction rate in this domain. Onthe other hand, the slope of the data points to the right of thevertical line is about 3600° K., corresponding to an apparent activationenergy of about 30 kJ/g-mol. Since the activation energy is greater thanabout 12 kJ/g-mol, it is believed that the intrinsic surface reaction iscontrolling the kinetics in this regime.

In summary, based on the experimentally-determined value of the apparentactivation energy, it is believed that the rate of atmospheric hydrationof the subject burnt magnesite is limited by pore diffusion and masstransfer at temperatures between about 140° F., and 180° F. In terms ofprocess and energy economics, this is a disadvantage since, in general,the rates of processes limited by pore diffusion or mass so transferincrease only as a fractional power of temperature. The overall resultis a diminishing return on higher hydration temperatures and externalenergy costs.

No claims are made in the present invention with regard to understandingthe mechanism underlying the efficacy of pressure hydration inshortening hydration time. It appears likely, however, thatsuper-atmospheric pressures can provide the motive force (pressuredifferential) for "pumping" after through the labyrinth of pores inwhich the large majority of reactive MgO surface resides. With the porediffusion and mass transfer resistances overcome, the intrinsic surfacereaction (chemical conversion of MgO to Mg(OH)₂) probably controls thehydration rate. In this situation, the rate of reaction generallyincreases exponentially with increasing hydration temperature. Thegreater rate of return on external energy costs and process economicsare thus probable advantages of pressure hydration over atmospherichydration.

Other embodiments of the invention will be apparent to those skilled inthe art From consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only with the true scope and spiritof the invention being indicated by the following claims.

We claim:
 1. A process for the production of a stabilized magnesiumhydroxide slurry comprising the steps of:.Iadd.providing chloride ionsto a mixture comprising burnt natural magnesite and water and then.Iaddend.pressure hydrating .[.a.]. .Iadd.the .Iaddend.mixture.[.comprising burnt natural magnesite and water.]. to provide aprecursory pressure hydrated magnesium hydroxide slurry; providing.[.chloride ions and.]. cationic polymer .Iadd.to the precursorypressure hydrated magnesium hydroxide slurry.Iaddend.; .Iadd.and.Iaddend. deagglomerating said precursory pressure hydrated magnesiumhydroxide slurry .[.in the presence of said chloride ions and cationicpolymer.]. to provide a deagglomerated and stabilized magnesiumhydroxide slurry comprising from 50 to 65 weight % solids. .[.2. Themethod of claim 1, wherein said chloride ions are provided prior topressure hydration..].3. The method of claim .[.2.]. .Iadd.1.Iaddend.,wherein said chloride ions are provided in an amount ranging from 0.01to 2.5 weight % of dry MgO.
 4. The method of claim .[.2.]..Iadd.1.Iaddend., wherein said chloride ions are provided in an amountranging from 0.2 to 0.5 weight % of dry MgO.
 5. The method of claim 1,wherein said chloride ions are provided from at least one memberselected from the group consisting of calcium chloride, sodium chloride,aluminum chloride, potassium chloride, magnesium chloride, ammoniumchloride and hydrated salts thereof.
 6. The method of claim 1, whereinsaid chloride ions are provided from calcium chloride.
 7. The method ofclaim 1, wherein at least a portion of said chloride ions are providedby anions associated with said cationic polymer.
 8. The method of claim1, wherein said cationic polymer is provided in an amount ranging from0.01 to 2.0 weight % of slurry weight. . The method of claim 1, whereinsaid cationic polymer is provided in an amount ranging from 0.1 to 0.2weight % of slurry weight.
 10. The method of claim 1, wherein saidcationic polymer is a polyamine.
 11. The method of claim 1, wherein saidpressure hydration is conducted at pressures ranging from 1 to 150 psig.12. The method of claim 1, wherein said pressure hydration is conductedat pressures ranging from 25 to 100 psig.
 3. The method of claim 1,wherein said deagglomerated slurry comprises from 57 to 62 weight %solids.
 14. The method of claim 1, wherein said burnt magnesite isfinely divided and capable of passing through a 20 mesh screen.
 15. Themethod of claim 1, wherein said burnt magnesite is finely divided andcapable of passing through a 100 mesh screen.
 16. The method of claim 1,wherein exothermic heat associated with the hydrating step is recovered.